Well, before the Emperor started the creation of Space Marines as we know them, he created 20 Primarchs. Born from genetic engineering and a bit of Warp (magic), the Primarchs are basically the pinnacle of a human being. Super strong, smart, powerful. They are to a space marine what a space marine is to a human, in terms of abilities.

While the Primarchs were still in embryonic state, the Chaos Gods scattered them across the Galaxy, forcing the Emperor to look for them everywhere.

In the meantime, he used the Primarchs' genetic material to create the Space Marines. As there were 20 Primarchs, so did the Emperor form 20 Legions of Space Marines, each Legion's warriors sharing some genetic traits with their Primarchs (Space Marines refer to their respective Primarch as gene-father).

Hope that helped. If you can, do read more on the subject, this stage in Imperium History is awesome.

It takes a long time to get the right pose and even then after you are done with just the sketch, that it looks like crap. I know the feeling. Most times, I end up not finishing a sketch I just started because a part of the pose doesn't look right.

Aye, it used to be my modus operandi, pretty much, I still have a swell number of 'to-be-continued' pieces, but I easily lose the will to finish them, so I'm trying to finish all of my artwork in a go, shaping my mind as well as my skills.